(1) Field of the Invention
The present invention relates to the launching of aerial vehicles and is more particularly (though not exclusively) concerned with the launching of high altitude, long endurance, solar powered, winged, unmanned aerial vehicles (UAVs).
(2) Description of the Art
Solar powered UAVs have been proposed for use as long endurance aerial platforms, flying at stratospheric altitudes (typically between 15,000 and 30,000 m), for such roles as communications relay, earth observation, weather monitoring, surveillance and mapping. In this respect they may provide a cost-effective alternative to the earth-orbital satellites conventionally used for these purposes and have the additional advantage that they can be easily controlled to remain in sight of a selected part of the globe or to fly to any other selected station. Such vehicles comprise a lightweight structure including at least one aerodynamic lifting surface (wing), one or more electrically-driven thrust-producing motors, an array of photovoltaic cells, and electric storage means such as rechargeable batteries or regenerative fuel cells. An example of such a vehicle is described in U.S. Pat. No. 5,810,284, WO 01/58756, WO 01/58757 and WO 01/58758. In principle vehicles of this kind can remain aloft indefinitely, limited only by the reliability of their mechanical components and airframe .integrity, by following a diurnal flight pattern in which, during the hours of daylight, the power generated by the photovoltaic cells is used to drive the motors at high power to climb the vehicle from a base operational altitude to a higher operational altitude and to charge the electric storage means, and during the hours of darkness the motors are driven at lower power from the storage means and the vehicle is allowed to descend to the base (though still stratospheric) operational altitude.
Atmospheric conditions within the stratosphere are generally benign, meaning that an airframe designed to fly only within the stratosphere can be less robust, and hence lighter, than one designed to fly within the variable atmospheric conditions of the troposphere. Furthermore the minimisation of airframe weight is of especial importance to the success of a long endurance vehicle whose power supply is limited to the electricity which can be generated from onboard photovoltaic calls during daylight.
The conventional method by which a high altitude, solar powered UAV is initially launched to its operational altitude—such as the one described in U.S. Pat. No. 5,810,284 et al —is for it to climb under its own power from the ground. This implies, however, that the airframe must be sufficiently strong (and therefore heavy) to cope with the additional loads imposed by flight through turbulent regions of the troposphere (hence imposing an undesirable limitation on the payload which it can carry) and/or that it will risk airframe damage during the climb and/or that its launch must await the most favourable weather conditions.
U.S. Pat. No. 4,697,761 discloses an alternative method of launching a solar powered UAV in which it is carried aloft, suspended in a 90° banked attitude, inside the envelope of a lighter-than-air balloon. When a desired altitude is reached the lower end of the balloon is unreefed and, after a brief delay, the UAV is released and free falls from the balloon before assuming level flight under its own power. No details are provided of aerodynamic control required to transition from the free fall 90° banked attitude to level flight. It is likely, however, that significant loads would be imposed upon the airframe in recovering from the drop from the balloon, meaning again that it must be stronger (and therefore heavier) than that required for normal stratospheric flight.